The PD1 / PD2 photobridge acts as a voltage
divider with the midpoint at Vcc / 2 when the light
on each PD is equal. The response is linear over a
wide range of ambient light levels. Some PDs are
very efficient and may need some light shielding
(i.e. heat shrink) to reduce the photo
current level in very bright light.

The hi / lo oscillator uses R1 / C1 to set the
basic frequency. Resistor R3 connected from the
photobridge midpoint to the R / C node of the
oscillator is used to influence that frequency.
That frequency depends on the voltage of the
midpoint as well as the absolute resistance of the
PDs. In general, the frequency is the lowest when
the PD bridge center voltage is near the threshold
of the 7AC240 inverter input.

With an R1 to R3 = 10 to 1 ratio, the frequency
and the duty cycle varies when the PD output
voltage is within 10% (+ / -) of the threshold.
Voltage levels above and below that +/-10% range
will cause the output of the oscillator to stop and
will be steady high or low, inverted with respect
to the input voltage. The band of oscillation can
be made more or less by decreasing or increasing
the ratio of R1 and R3 values (i.e. if R1=R3
it always oscillates). I use typical ratios of
between 2 to 1 and 10 to 1. If the ratio is high,
the sensitivity is greater but the tendency to
wiggle is also greater. Remember a shaky head is
not power efficient.

For some applications that have lots of build in
damping (i.e. Submarine head, Mazola head)
R3 can be 0 ohms ;).

The Nv / Nu driver (also called Nx driver) both
differentiates and integrates the complementary
outputs from the oscillator. When the oscillator
oscillates, the Nx driver AC couples the output
pulses noninverted as the Nx time constant is
longer than the oscillator period. With AC coupled
(non-inverted AC) pulses in phase across the motor,
the PSHead is in the power save mode but with the
brake on! That is important to be able to stop the
motor rotation quickly to avoid mechanical
overshoot when the head is aligned with the light
source.

When the oscillator stops oscillating theres are
no pulses to AC couple and instead, the steady
output is DC coupled (and inverted) to cause a
differential voltage across the motor which then
rotates.

The ratio of the R1 / C1 and R2 / C2 time
constant should be about 1 to 10 for low standby
power. Smaller ratios can be used for faster and
more "varied" response but sometimes higher standby
current. Since the R1 / C1 time constant is
influenced by R3 and the photo bridge output
voltage, that ratio will vary with light level. The
ratios of those two RC time constant component
values cause a variety of behaviour, sensitivity
and efficiency.

For solar powered PS Heads, the time constant of
R2 / C2 should (probably) be much shorter than the
duration of the SE pop or some unpredictable
(perhaps interesting) side effects may occur.

This PowerSmart head circuit was solarized, courtesy of
Darrell Johnson, and dubbed the Solar Power Smart Head
(SPSH). The SPSH circuit has been tweaked from time to time
(each time improving its efficiency); the most recent update
posted
by Wilf is shown here:

Circuit schematic (click to
enlarge)

Circuit layout (click to enlarge)

There's also a corresponding 2 degree of freedom (i.e.,
for a 2-axis head) version of this circuit:

Circuit layout (click to enlarge)

Recently, Wilf posted
version 3 of the SPSH -- this time with an LED flasher
circuit. Here's Wilf's explanation:

I checked some RS FLEDs I have and the average
current is about 2 ma. I designed a new 74HC240 LED
flasher circuit which has an average current of about 100
uA. The LED flashes much brighter than the FLEDs. The
circuit provides some voltage boost so it works down to
1.5V. The low power LED flasher comes on when the SE
triggers and stays on until the SE resets. The rest of
the SPSH layout has been changed somewhat to simplify
connections between IC pins. A SPSH4 design is next that
replaces the 1381 with two transistors and provides dual
motor drivers.

Circuit schematic (click to
enlarge)

Circuit layout

For more information...

Make sure to check out Wilf's writeup on powersmart
heads, BEAM
Heads 101.